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Subsidence Evaluation For Exhibit 60B South of Divide Mining Area Page 7 cracks in zones of convex downward curvature (Figure 2). Fracturing within the expected zone of fracture may cease completely where soft shales and claystones occur as alternating sequences with sandstones. The maximum height of fracturing above longwall panels in the South of Divide mining azea is estimated to range from about lOt to 20t. This is neaz the mid-range of 9t to 30t as reported by Peng (1992, p. 7). This estimate maybe conservative for the particular rock strata or lithology above the E Seam. Drainage, however, may cease after mining is complete and any water bearing zones present may be restored. This is particulazly likely in the upper part of the fractured zone in shale sequences between sandstone layers, once subsidence is completed and the sepazated beds re-compress and close in response to overburden load (Figure 2). Evidence of restored water levels has been measured and reported in some wells in the West Elk Mine subsidence monitoring azea after mining and subsidence were complete. 5.3 Continuous Deformation Zone and Near Surface Zone These two zones are discussed together because the ground surface is where neazly all measurements aze made that monitor subsidence processes active in the zone of continuous deformation. The neaz surface zone, which typically consists of weathered bedrock, colluvium, alluvium, and soil a few feet to a few tens of feet thick, may deform differently than the underlying bedrock (Figure 2). Field studies by the author indicate that neaz-surface colluvium and alluvium, which consist of predominantly clay and silt, can undergo significantly more extension without rupturing than can the underlying material. hi both the Somerset, Colorado and Sheridan, Wyoming azeas colluvium and alluvium 5 to 10 feet thick were observed to cover cracks as much as 10 to 14 inches wide so that there was no indication of the underlying ruptures. The zone of continuous deformation, which is transitional to the overlying neaz-surface zone and also to the underlying zone of fracturing, undergoes differential vertical lowering and flexure as laterally-constrained plates (in three dimensions) or beams (in two dimensions). With flexure, sheaz occurs at the boundaries of rock units with different strength and stiffness, chazacteristics, such as sandstones and shales. Zones of tension above the neutral surfaces of a rock unit, for example, become compressive above the boundary with another rock unit and below its neutral surface (Figure 2, Enlazgement 2). Any cracks, therefore, which occur in the tension zone of a rock unit, temvnate at the neutral surface, because the unit is in compression below this point. 5.3.1 Vertical and Horizontal Displacement, Tilt, and Horizontal Strain Differential vertical lowering of the continuous deformation and near surface zones causes vertical displacement (S), horizontal displacement (S~, tilt (Ivi), and horizontal strain (E). In flat or gently sloping terrain (slopes less than about 30 percent), surface profiles of subsidence • depressions aze similaz to flexure of fixed-end, laterally constrained beams. Tensile stresses aze 831-032.620 Wright Water Engineers, Inc.